In the original ballot held in May, Austria came within 31,000 votes of becoming the first western European country to elect a far-right head of state since World War II.

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Norbert Hofer, of the anti-migrant Freedom Party (FPO), lost that vote by less than one percentage point to independent Alexander Van der Bellen, a former leader of the Green Party.

The Constitutional Court ordered a re-run after a challenge by the FPO, with judges citing irregularities in the way some postal ballots were processed.

The court decision was seen as a victory for the FPO, giving it more time to exploit widespread anti-migrant sentiment in favour of its candidate. In recent weeks, polls have given Hofer a four to six percentage point edge over Van der Bellen.

Austria's president has mostly ceremonial responsibilities.

A Hofer win, however, could be seen as a boost not only for his party but also for far-right and nationalist movements elsewhere in Europe which all are lobbying for a weaker European Union or an outright exit from the bloc.

With no president now in office, the post's functions are being exercised by the three parliamentary presidents, one of whom is Hofer.